Avoid playing NPCs that accompany the PC on their adventure. The player can get in a rut of relying on the GM and their "magic NPC" to get them out of trouble.
Consider letting the player play multiple characters. There's nothing wrong with games where the player says something like "The Doctor will build the anti-Dalek doohickey while Martha sneaks into the prison block to free the slaves." Such games will be spoken less in the first person than games where each player plays only one character, but that's not really a problem.
Be generous with story points, but also be severe in consequences. Story points should be flowing like water between player and game master when there is only one player. But it should never be easy: the player should actually need those story points to survive.
Don't get into a lot of combat situations, especially where it's many enemies against the player character.